Nitriding with electric glow discharge



United States than c lQQ NITRIDING WITH ELECTRIC GLOW DISCHARGE Bernhard Berghaus and Hans Bucek, Zurich, Switzerland, assignors to Elektrophysikalische Anstalt Bernhard v Berghaus, Vaduz, Liechtenstein This invention relates to the production of improved surface layers on tubes of iron, steel or iron alloys, more particularly gun tubes. Owing to the great stresses-to which such tubes are exposed by corrosion, erosion, firing, etc., their surface is quickly attacked and destroyed. More particularly gun tubes, owing to rapid repeated firing, are subject to considerable wear, so that they become useless after a short time and have to be replaced.

, It has already been proposed to avoid these drawbacks by making the tubes of highly alloyed steel. However, frequently the cost of such tubes is not justified by the advantages thereby obtained. It has also been proposed to so treat the surfaces exposed to wear that they become more resistant thereto. For instance, it has also been proposed to nitride these surfaces. However, none of these endeavours has led to a useful result. It has further been endeavoured to subsequently chromium-plate such nitrided surfaces, whereby some progress could be made, but this process was too cumbersome and difiicult to carry out.

In contradistinction thereto, it is possible, according to the invention, to increase the resistance of the tubes substantially, by improving them through ion bombardment in a glow discharge; It is possible to use ions of nitrogen, carbon, silicon, boron, metal, hydrogen and others, alone or in admixture with'one another. This treatment is characterised by the feature that use is made as a means for heating up to the required reaction temperature, of a glow discharge at any desired gas pressure. When use is made of high gas pressures of 2 mm. Hg and more, the electric glow discharge is preferably effected by periodic supply of energy of short duration, whereby the energy pulses of short duration are at least 10% higher than the permanent voltage. The intervals of rest are thereby equal to, or greater than, the periods of time of the increased energy supply. The glow discharge is thereby operated with about 0.2-5 Watts per cm?v of the surface of the ,object to be treated. Other gases,'preferably hydrogen, may be admixed to the gas'used for the treatment. When the latter is nitrogen, it is especially advantageous to add to it hyrogen in the ratio of 1:10 up to 1:25. Before their introduction into the treating chamber, the treating gases are purified, in order to remove also small amounts of foreign gases, e.g. oxygen, or when working with nitrogen for instance, also small amounts of hydrocarbons. This purification of the treating gases may advantageously be effected by means of ferrosilicon, ferroaluminium or other absorbent at higher temperatures of about 600-1000" C. The treatment is preferably effected with flowing gases, whereby the effective amount of gas should be equal to, or greater than, the consumption in reaction gas. It is an'advantage to carry out the treatment until the improving layer has reached a thickness of about ,4 mm. or more.

According to the invention, the tubes are connected as a cathode of the glow discharge. The leads to the electrodes are insulated and screened with respect to the gas discharge by means of a gap.

In order to facilitate the entrance of the glow discharge into the interior of the tube, a wire may be disposed in the axis of the tube, to which a voltage is also applied. In order to carry out the process according to the invention, the total electric power that is applied should be at least, 3 or 5 or 10 kw. Further, according to the invention, the articles to be improved are subjected, prior to the actual treatment, to an ion bombardment of reducing gases, such as hydrogen or the like. Thereby, it is an advantage to maintain the articles at a high temperature by means of a glow current. This preliminary treatment improves the technical properties of the material for instance, by the removal of small amounts of sulphur or phosphorus, e.g. with respect to the impact resistance proving elements.

For instance, a gun tube is verticallysuspended in a etc., and prepares the surface of the articles to be treated in a substantial way for the reception of the actual imgas discharge chamber and connected to the negative pole of a source of DC voltage, e.g. a rectifier, capable of supplying a current of a few amps. in 'a glow discharge at a voltage of a few hundred volts, which glow discharge takes place in a gaseous atmosphere at a pressure of, for instance, a few mm. Hg. The positive pole of the source of voltage is connected to an anode introduced in a gas discharge chamber or to the metal wall of the gas discharge chamber. Thus the gas discharge takes place between the gun tube acting as a cathode and the part of the anode connected to the positive pole of the source of voltage.

Use is made of a gas containing nitrogen which has been purified by means of ferrosilicon at high temperature before introduction into the discharge chamber, for instance a mixtureof nitrogen and hydrogen or ammonia gas, 'or ammonia enriched with nitrogen, or diluted with hydrogen or another suitable gas, for instance a rear gas. 7

Now, such an electric voltage is applied, and the gas dis-' charge pressure is so, adjusted, that the tube assumes the temperature suitable for hardening by nitriding which, as a rule, is about 500 to 550 C; The duration of'the treatment depends on the desired depth of nitriding, being about 12 to. 30 hours, but it maybe greater orvsmaller according to special requirements. 'By this process one attains, in addition to other advantages, a much quicker depth penetration than with the process hitherto usual.

For instance, an ammonia gas. is produced with a pressure of 0.6 mm. Hg in an evacuated metal vessel in which the gun tube is suspended, the positive pole of a' DC.

source of voltage. being connected to the wall of the discharge. This screening is advantageously effected by' the provision of a narrow gap between the conductors of different potential in such a way that the gap, seen from the interior of the vessel, is in front of the insulation, so that the glow cannot reach the insulating material.

After treatment of about 24 hours, the tube has a very hard surface layer, which is not brittle and,therefore, does not tend to splinter oif, and is thus characterized by a high degree of ductility despite its increased hard ness.

It is advisable in many cases, especially when working at a high pressure, to supply the glow discharge energy by way of impulses, that is to say, to use an electrical energy for a longer period of time, which would produce substantially higher temperatures of the gun tube than are desirable if applied continuously, but to apply this energy only for short periods of time and insert between these times periods of lower voltage, which are approxib Patented July 26, 196i) mately as long as, but preferably substantially longer, than those during which the energy of high intensity is applied. For instance, there is applied a permanent voltage of 400 volts and at intervals of 3 to 5 seconds,

lasting a few tenths of a'second voltages are applied whichare a multiple of, for instance ten times the permanent voltage. The gas pressure can then be increased to a few mm. Hg. A special advantageof this periodical treatment lies therein that the temperatures of the tube can be kept so low that the quality of the steel is not thereby.

in any way influenced. A further advantage lies therein that the glow discharge acts with suflicient intensity also on the parts of the tu e which are most removed from the ends of the tube, whereby a more uniformly nitrided surface is obtained.

The uniform treatment may be assisted by the provision of a wire in the the axis of the tube, to which the elec tric voltage is also applied. According to circumstances, the wire may .beconnected to the positive or negative pole of the voltage. If such a wire is connected to the negative pole the wire is disintegrated and the material that is disintegrated may be deposited on the interior of the tube or with a longer period of time be diffused therein. The high temperature thereby required on the inner surface of the tube may be obtained by supplying the energy periodically for sufficiently short periods of time, as above described. By using a suitable gas pressure with respect to the distance of the central wire from the inner surface of the tube a discharge is obtained in which very high energies are released.

The above details concerning the process are given only by way of example. They may be varied according to circumstances, or be supplemented; they have to be adapted to the extent of the use, the size of the tube, the internal diameter of the tube and other circumstances. As a rule, such adaptation can be effected by changing the applied voltage, the gas pressure, the composition of the gas, and sometimes by adjusting the periodical pulses of energy by using electrodes in the interior of the tube and applying a'suitable voltage to an electrode.

The glow discharge chambers that mainly come into question are metal chambers which are sufiiciently long to accommodate a tube and have a sufificiently large diameter to accommodate one or more parallel tubes without disturbing one another. Although it has been found an advatage to suspend a tube in the gas discharge chamber, the invention is not limited to such an arrangement.

What I claim is:

1. Process for hardening the interior surface of steel gun-barrels which comprises subjecting the portion of the interior surface of a gun-barrel which is contacted by the projectile to an electrical glow-discharge in a nitrogenous atmosphere to effect heating of the interior surface of the gun-barrel, substantially entirely by such glowdischarge, to a nitriding temperature and heavy bombardment of such surface, with nitrogen ions to enrich 4. Process according to claim 1, wherein the glow dis-' charge is produced by a permanently applied voltage on which there are superimposed periodic pulses of short duration of a voltage at least 10%higher than, and up to ten times, thepermanent voltage.

5. Process according to claim 1', wherein a conducting wire is disposed axially within the gun-barrel and is connected to a source of electrical tension.

6. Process according toclaim 1, wherein the gas pressure is below atmospheric and wherein the electric glow discharge is effected by means of periodically supplied energy pulses at difierent voltages and of relatively short duration.

7. Process according to claim 1, wherein the glow discharge is effected by the expenditure of electric power at a density of 0.2 to 5 watts per cm. of the surface being treated.

8. Process according to claim 1, wherein the gaseous atmosphere contains hydrogen mixed vn'th nitrogen.

9. Process according to claim 8, wherein the proportion of nitrogen to hydrogen is from 1:10 up to 1:25.

10. Process as claimed in claim 1, wherein the interior surface of the gun-barrel is subjected to an ionic bom bardment in a reducing gas prior to the treatment with nitrogen.

11. A gun-barrel produced by the method defined in claim 1, wherein at least those parts thereof which are brought into contact with a projectile possess a superficial zone which is enriched with nitrogen and, while possessing a hardness at least equal to that achieved by thermal nitriding, is of greater ductility, is more resistant to wear, and possesses an increased resistance to splintering.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Achiv fiir das Eisenhuettenwesen," Bench and.

Ruediger, vol. 18, July-August 1944, No. 12, pages 61-67. 

1. PROCESS FOR HARDENING THE INTERIOR SURFACE OF STEEL GUN-BARRELS WHICH COMPRISES SUBJECTING THE PORTION OF THE INTERIOR SURFACE OF A GUN-BARREL WHICH IS CONTACTED BY THE PROJECTILE TO AN ELECTRICAL GLOW-DISCHARGE IN A NITROGENOUS ATMOSPHERE TO EFFECT HEATING OF THE INTERIOR SURFACE OF THE GUN-BARREL, SUBSTANTIALLY ENTIRELY BY SUCH GLOWDISCHARGE, TO A NITRIDING TEMPERATURE AND HEAVY BOMBARDMENT OF SUCH SURFACE, WITH NITROGEN IONS TO ENRICH THE TREATED SURFACE WITH NITROGEN THROUGH THE GLOW-DISCHARGE UNTIL THERE IS OBTAINED A HARDENED INTERIOR SURFACE ZONE CHARACTERIZED BY INCREASED RESISTANCE TO WEAR, AND BY DUCTILITY AND RESISTANCE TO SPLINTERING. 